Cascaded multi-phase electric drives are used in industry to provide variable electric power to AC motors. These same drives can be used in other applications not related to specifically to motors but where a variable-output voltage or frequency is desired. Typical drives have an AC input power source and some type of conversion apparatus, usually using solid-state devices, for converting the fixed AC input voltage into a variable-voltage and/or variable-frequency output. One such type of drive is described in U.S. Pat. No. 5,625,545, which is incorporated herein by reference. That patent describes a power supply used as a drive which utilizes a number of power cells (power units) arranged to produce a three-phase AC output. Such multiple power units in series can be utilized to provide higher voltage outputs than would be available with only a single power unit. In the power cell, the DC currents of the rectifier generally will match the DC current of the output converter, but the instantaneous ripple currents generally will not match. It is preferred to provide smoothing capacitors, which can act as a current smoothing filter, to carry the difference ripple current. Capacitors are representative of a capacitor bank, the precise values of which may depend upon the power requirements of the inductive load.
The DC capacitor bank of power cells using intermediate voltages, for example, above 400 V, often use two or three electrolytic capacitors in series. If two level inverter topologies are used, the voltages of the intermediate potentials between those capacitors are not measured.
When electrolytic capacitors are overstressed they can become thermally unstable, which can cause violent explosions driven by evaporated liquid electrolyte. Such explosions must be avoided due to safety and secondary damage. Monitoring the overall voltage of the series connected capacitors is not sufficient since unequal parameters of the individual capacitors due to aging or defects can cause excessively unequal voltage sharing among the series connected capacitors.
Therefore, monitoring the voltage of the individual capacitor among the series is highly desired.